I think there are a couple of different ideas running through this thread and I think I may be somewhere in the middle.
#1 Should an aikido instructor tell a prospective student that if they learn aikido, they will be able to do certain things in terms of self defense?
By my way of thinking - No. I believe its not the art that makes you but rather it is the individual that makes the art. No one can promise that the study of any art will make them anything. That depends on the diligence, perseverance, type of teacher, types of training partners, overall coordination and athleticism of the practitioner, his or her height, weight, strength and ability.
Having said that, I would say exactly the same thing for any other art including BJJ, Kung fu, Judo, or Karate. They get the same kinds of people that we do. I think it is OK for an instructor to tell a prospective student what aikido is designed to do such as defend against multiple opponents. You can say that it has both katame waza, nage waza, combination techniques, reversals and changes (henka) and floor techniques. I think it's fruitless to compare it to other arts because that involves intangibles that no one can measure beforehand. I would emphasize that this art is one that emphasizes removing a fighting spirit and personal transformation. If you just want to learn to fight, another art might be better for you.
Secondly, what is aikido's potential. That potential in terms of self defense is limitless. I don't think aikido takes a back seat to any other art. Again though, it depends on the factors I previously mentioned. If you take a big, strong, fast 6th dan in Aikido and put him against lessor endowed BJJ man, the Aikidoist will prevail. If you pit him against a similarly ranked practitioner of any other art, then we can't tell because there are too many intangibles. Remember, Buster Douglas defeated Mike Tyson. I can say one thing though, the Aikidoist would try to stay out of that fight and if he was really high ranked, he would win because that fight would never happen.
Lastly, I don't think a person should cross train until he or she has completely learned the basics of their art and developed that somewhat. I also don't think that a person should cross train trying to round themselves out or add to themselves what they lacked in another art. Any art takes a lifetime to master and diluting your time and efforts like that will make you a jack of all trades and master of none.
Having said that, you should know that I cross train in two other arts besides Aikido. I train in Daito ryu Aikijujutsu and Iaido. I didn't start training in these other arts until I was almost 3rd dan in Aikido. I also didn't go to these arts out of any dissatisfaction with Aikido nor to become a "better fighter" or to improve my self defense skills. I train in Iaido because I am fascinated by the kenjutsu my Aikido sensei teaches us. I want to better understand the relationship and approach of a different style compared to our own so that I can be more proficient and knowledgeable in what we are doing with the Aikido kumi tachis. As in any discipline, a broader perspective helps you understand your own perspective better. For example, I am a Christian but I have spent a lot of time studying world religions. I wasn't dissatisfied with my own religion. I felt no lack in my personal practice of Christianity. I wasn't seeking to mix all these belief's together into some form of syncretism.
I was seeking to see the whole picture better and to establish my own beliefs against a context. That is why I cross train in martial arts.
As for Daito ryu, it is the parent art of Aikido. I am doing it for the same reasons. I am interested in the different approaches to similar concepts such as kokyu-ho and aiki.
In terms of class time, I train 52 hours a month in Aikido, I train 4 hours a month in Iaido and 4 hours a month in Daito ryu aikijujutsu. That should show where I think a person's priorities should be in terms of Aikido and other martial arts. This one is hard enough to learn!
I don't think a person should be filled with self doubt about their martial art. I have none about Aikido. I have seen too many wonderful and powerful people in Aikido. Should we exaggerate? No. Can we trust that this art can handle most contingencies? Yes, but under certain qualifications that are also apply to all martial arts. If you can't defend yourself with this one, I suspect you will have the same problem somewhere else that you had here. You might want to look into handgun training and get a concealed weapons permit. That should solve your problem - unless your attacker has a bullet proof vest!